USB device stack

Dependents:   mbed-mX-USB-TEST1 USBMSD_SD_HID_HelloWorld HidTest MIDI_usb_bridge ... more

Adjusted the stream name of USBSerial

I named the USBSerial stream 'USBSerial' in the constructor. Since there always is only one USB controller I didn't see a reason to make this naming public. Now the USBSerial can be used with freopen()

USB device stack with USBSeral Stream Named device, USB